This must be "hot button" question week.
First, you are correct in understanding that commissions are negotiable. BUT, each and every brokerage you interview will be taking into account the fact that there are some fixed costs with marketing your property, and also there are fixed costs to protect you, the homeowner, from legal tangles that can arise from dealing with a difficult buyer, or a buyer who comes to get you over time because you may not have disclosed a significant issue when the home sold.
Now...you must also remember, that selling and marketing your home takes a bit of knowledge about the local market that agents will have to have. If you negotiate low enough, you will get the lowest common denominator - in other words - you'll get what you pay for. If you get an agent that caves in to your demands for the lowest commission, then you get what you pay for. Will that agent be the best (guess what, good agents with good reputations command good commissions) for your needs? Or will you negotiate low enough to be attractive to work with from an agent starting out, or one that is not so well versed in marketing your home? Will you negotiate low enough to make an unknown difficult effort worthless of the agent's commensurate fee? Also, under current trends, it helps to make sure you have an edge in marketing and salesmanship - if you go for the lowest possible margins, you will get the lowest possible efforts.
I think if you want to start at the bottom and work up, look into what Help-U-Sell can do for you. They don't ask for the highest commissions, and what they will do is send you a sales "kit". They will put your home on the MLS: you (basically) will do the rest (you're own open homes, make your own property statements, provide them with photos, complete the disclosures without too much consultation, you'll get almost NO consulting on how to prepare your home, how and why you should stage it, and no consultation on what reports you should order and how much they are). If you're up for the pioneering effort, then this is a good way to go.
Most agents and brokerages are full scale, full effort, full service. That means they will gladly share their experiences with you, along with their contacts for doing work. If you want a treat, hire a full service brokerage and tell them what you want and under what budget you want to give them. Let them run with the plan you devise together and you should be surprised what you get for the fee. In order for you to get the best dollar for your home, you're going to have to think about the fact that a more generous effort on your part gleans the very best efforts of all those that want you to succeed in getting those highest dollars!
I wish you luck and hope you find the agent that fits your needs. You DO get what you pay for.